This invention relates generally to a system for ruling diffraction gratings whose groove spaces replicate the gratings of a model diffraction grating, and more particularly to a system for ruling a grating blank with blazed grooves so as to have the properties of a holographic grating by using an aberration corrected holographic grating as a model.
The fabrication of diffraction gratings has heretofore utilized either of two techniques; namely, mechanical ruling and holographic recording. Each had its own particular advantages and disadvantages. Mechanical ruled gratings are typically fabricated by driving a diamond stylus across a coating of metal or other material formed on a polished substrate and thereby burnishing a groove in the surface. This process is carried out with great precision in a step and repeat process with the result being a grating having generally straight, parallel and evenly spaced grooves.
Holographic gratings, on the other hand, are fabricated by coating a polished substrate with photoresist material and thereafter exposing the photoresist to an optical fringe pattern produced by the interference of two coherent light beams and which is thereafter followed by developing the exposed photoresist material. The result achieved in holographic grating is a corrugated surface corresponding to the fringe pattern to which the substrate was exposed.
The advantage of mechanical ruling is primarily in the ability to control the groove shape through the selection of the geometry of the ruling stylus and other ruling parameters because groove shape strongly affects the diffracted efficiency from the grating. The disadvantages of a mechanically ruled grating stems from the mechanical nature of the process itself. As the step and repeat process is carried out, random and periodic errors in the mechanical system act to perturb the groove-to-groove phase accuracy. The resulting undesirable effects of imperfect phase accuracy across the grating produce ghost images of the diffracted orders in the plane of dispersion and scattered light between orders in this plane. Both of these effects thus limit the usefulness of ruled gratings in modern relatively high sensitive, high resolution spectrographs. Holographically generated gratings, on the other hand, provide an advantage in that all the grooves are in perfect registration with each other because of the ideal behavior of the interfering light beams. All the grooves, moreover, are recorded simultaneously, which eliminates problems associated with mechanical errors. The resulting effects are the elimination of ghosts and very low scattered light in the plane of dispersion.
It should be noted that the groove shape of a properly blazed grating is triangular in vertical cross section and resembles a sawtooth, while the groove shape of holographic gratings is somewhat rounded and thus is approximately sinusoidal, hereinafter referred to simply as sinusoidal. The triangular groove is desirable in one respect in that it yields a relatively high diffracted efficiency into the desired order. The sinusoidal groove of the holographic gratings, on the other hand, exhibit an undesirable characteristic in that it leads to significantly lower diffracted efficiency into a desired order.
Accordingly, it is an object of the present invention to provide an improvement in the fabrication of ruled diffraction gratings.
It is another object of the invention to fabricate a ruled diffraction grating having the properties of a holographic grating.
It is still another object of the invention to produce a ruled blazed diffraction grating having the properties of an aberration corrected holographic grating.